Since having our new windows installed, I can leave the heat turned off all day, and the room temperature will dip by only .5 to 1 degree (10 degrees colder outside). With the old glass, the house would have gotten down at least 3 or 4 degrees, if not more.
In fact, last night when Heat Junkie Hubby came home, I forgot to turn the furnace back on (usually I scoot over to the thermostat when I see his car pull in the driveway ;-) He gets home at about 5:30, and it wasn't until the kids' bedtime that I realized I'd forgotten. Meanwhile DH hadn't said a word, 'cause he was still comfortably warm.
Now the unfortunate part is that we have to pay for the windows (not like we can return 'em!!). Over the long term, though, they will save us money (plus the house looks so much nicer :-) !!